In display devices using LED (Light Emitting Diode) elements as display pixels, in order to perform matrix drive operation of the LED elements, X-Y addressing drive circuits are required for respective pixels. The display device serves to perform selection (addressing) of a LED element located at the position of pixel desired to be emitted (lighted) by addressing drive circuit to modulate lighting time by, e.g., PWM (Pulse Width Modulation) drive system to execute luminance adjustment to obtain display picture having a predetermined gradation.
However, when drive circuits are assembled with respect to individual LEDs, in the case where the number of LEDs is large, the circuit configuration becomes complicated so that cost is increased.
On the other hand, it is proposed and studied to use LED elements as backlight light source for liquid crystal display. Particularly, since a method in which LED elements of respective primary colors of red (R), green (G) and blue (B) are individually used to optically perform synthetic additive color mixture to obtain white light can easily take color balance, such a method is extensively studied as display device of television image receiver.
Meanwhile, LEDs individually have unevennesses of luminance values. When attempt is made to correct those individual unevennesses, respective individual elements must be necessarily driven, one by one, by independent drive circuits. As a result, drive form extremely becomes similar to that of the matrix type drive system corresponding to the previously described display device using LED elements as display pixels. Namely, in the case where the number of LED elements is large, drive circuit by addressing would become complicated.
Moreover, in the case where, e.g., LED elements are used, as light source, for backlight of liquid crystal display device, since light emission coefficients of LED elements of respective primary colors of red (R), green (G) and blue (B) are different from each other, it is necessary to also adjust, every colors, currents to be applied to LED elements of respective colors. Further, in the LED elements, since semiconductor compositions are different from each other every respective colors, voltages and power consumptions of elements are different from each other every respective colors.
In addition, in actual circuits having large powers of respective LED elements and used in LED drive operation for illumination purpose, since LSI, etc. for large power drive is not yet prepared, the cost is increased in the matrix type drive system so that it is economically disadvantageous.
In view of the above, there is proposed a method in which connection form of LED elements is used as cascade connection form in order that the circuit scale is not caused to be large. In the cascade connection form, PWM adjustment of currents in a certain series of LED connection groups, e.g., groups in which LED elements of red, green and blue are connected every respective colors is performed to adjust color tone and luminance based on synthesis of rays of light emitted from LED elements of red, green and blue.
In the backlight unit in which the cascade connection form is employed as connection form of LED elements, a DC-DC converter power supply unit for delivering a predetermined voltage every groups of red, green and blue LED elements which are cascade-connected is provided, and a LED-PWM control unit is provided at the load side.
Meanwhile, in the configuration as described above, since temperature dependencies of light emission outputs of respective color systems are also different and temperature characteristics are not uniform, it is necessary to perform adjustment of pulse width every colors by drive circuits dedicated for respective colors.
For example, under the situations where temperature is not completely elevated immediately after lighting of the backlight, the LED element of red having high light emission efficiency is emitted in a time of about 50% of ON time of drive pulse width of PWM signal, whereas the LED element having low light emission efficiency is emitted in a time of about 80˜90% of ON time of drive pulse width of PWM signal.
Since rays of light emitted from LED elements have such property, it is necessary for keeping constant color tone (color temperature and chromaticity) and luminance of white light obtained by synthesis of rays of light emitted from LED elements of red, green and blue to detect, by photo-sensors, rays of light which are respectively emitted from LED elements of red, green and blue to execute feedback servo so that the value thus detected becomes constant.
In such feedback system, e.g., in the case where resolution of change of pulse width for controlling PWM signal is coarse, there would result difference of adjustment accuracy such that, in dependency upon the number of divisions between 0% and 100%, change width becomes coarse in the case of the LED element of red having good (high) light emission efficiency, whereas change width becomes fine in the case of the LED element of blue having bad (low light emission efficiency)
Further, since colors of rays of light emitted from the LED elements have uneven accuracies every respective colors by differences of resolutions of respective color systems, adjustment of balance of RGB and/or adjustment of white light become difficult.
In addition, even if the above-described problems can be all solved, not only light emission output but also light emission spectrum distribution of LED elements of respective colors would change by temperature change in the LED elements of respective colors so that light emission chromaticities of respective colors change. Accordingly, in the case where there is only employed a method of detecting light quantities of LED elements of respective colors by the photo-sensors, it is impossible to correct change of color tone. In the case where the backlight unit has temperature distribution, e.g., in upper and lower directions with drive operation thereof, color unevenness based on difference of that temperature would take place. As stated above, by performance of the photo-sensor and/or temperature characteristic of light emission distribution of LED elements, it is a limit to maintain accuracy such that chromaticity control deviation is about Δx≈0.002 and Δy≈0.002.